1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device which is particularly applied to a gettering technique for removing contamination of metal such as heavy metal.
2. Description of the Related Art
High degree of integration and miniaturization of semiconductor devices lately tend to increase the number and kind of steps and facilities for manufacturing the semiconductor devices and accordingly semiconductor wafers are susceptible to contamination of metal such as Fe and Cu during the manufacture of the semiconductor devices. It is well known that the contamination in a depletion layer of PN junction causes a recombination current (hereinafter referred to as g-r current) which degrades electric characteristics of the semiconductor devices. The contamination particularly affects significant characteristics such as pause characteristics (charge retaining characteristics) of a dynamic random-access memory (DRAM) and, even though the contamination is less, it will become a serious problem in accordance with miniaturization of devices having a memory capacity of 16 M bits or more. The problem of contamination will be more and more serious. It is of course necessary that an influence of the contamination on not only the memory but also other devices be taken into consideration. For example, if a contaminant such as Fe is included in a silicon semiconductor substrate on which a charge coupled device (CCD) image sensor is formed, the contaminant is trapped and fixed in a defective lattice of a semiconductor wafer which is caused by ion-implantation after the wafer is annealed. When a region near a photo diode of the CCD image sensor in which the contaminant is fixed, is depleted, the contaminant is the main cause to generate electric charges and appears as a minute white stain (defective image which is whitely formed on a screen). The contaminant such as Fe and Cu causes the minute white stain to be generated. Although a reduction in the contaminant has been tried in respective manufacturing steps, the contaminant cannot perfectly be eliminated. It is thus necessary to use a gettering technique wherein the contaminant included in the wafer is effectively trapped in a region other than the active region of a semiconductor element.
The conventional gettering technique includes extrinsic gettering in which the backside of a wafer is damaged or distorted to getter impurities and intrinsic gettering in which small nuclei are formed by interstitial oxygen of a wafer and impurities are caught by the nuclei.
The following four methods can be used for the extrinsic gettering.
(1) The backside of the wafer is mechanically damaged or distorted by the honing technique using an SiO.sub.2 powder. It is the most common to use a backside-damaged wafer in which the damage or distortion is applied before the wafer is formed. PA1 (2) The backside of the wafer is damaged or distorted by ion implantation or laser irradiation. PA1 (3) Phosphorus is diffused into the backside of the wafer from a diffusion source such as PSG, POCl.sub.3, and phosphine formed on the backside thereof. (A gettering site is formed on the backside of the wafer by causing a misfit dislocation by the phosphorus.) PA1 (4) A film is formed to cause elastic distortion. PA1 a first step of forming a gettering site on a backside of a semiconductor wafer; PA1 a second step of subjecting the semiconductor wafer to a heat treatment to trap contaminant impurities in the gettering site; and PA1 a third step of removing a contaminated layer of the gettering site in which the contaminant impurities are trapped, PA1 the first, second and third steps constituting one cycle, and the one cycle being repeated at least twice.
The extrinsic gettering however has a drawback wherein the wafer is annealed by high-temperature heat treatment after the gettering and a function of the gettering site is reduced to half or lost. It is the latest mainstream that complementary MOS (CMOS) devices are used as semiconductor devices of low power consumption. It is well known that in the CMOS devices the gettering effect of the backside damage wafer is almost lost after a well is formed by heat treatment at about 1200.degree. C. Both the extrinsic and intrinsic gettering has a drawback wherein the contaminant trapped in the gettering site is easily freed therefrom by the heat treatment subsequent to the gettering and then diffused into an element active region on the surface of the wafer to thereby reduce the gettering effect to half.